moen shower mixer

I'm having a problem with my moen posi temp shower valve. So I have a tankless water heater, im in the process of fixing the elements right now so i can't actually test for hot water, but I can turn the water back on. I replaced the shower valve as it was giving me issues. Here's the problem...

Could anyone help me with the best solution to fix this PVC fitting on our shower mixer? I’m guessing I messed it up when changing the mixer cartridge. There’s the tiniest slowest leak dripping out the bottom. What be the easiest solution here? Thanks in advance

We’re starting the process of redoing our shower and I am unable to remove the shower mixer. All the screws were basically stuck, had to drill out the set screw for the handle but was able to remove it. Now I am to this brass piece, I have removed the screw in the center but it will not budge...

I'm having an issue (i think) with my Moen 1222 Positemp mixer being very erratic with the temperature of the shower from use to use. One day it'll work fine (mixes temp exactly in line with movement of the handle)... the next day we have to crank the handle all the way to the 9 o'clock...